Three layer stack structure

ABSTRACT

Vertically stacked system in package structures are described. In an embodiment, a package includes a first level molding and fan out structure, a third level molding and fan out structure, and a second level molding and fan out structure between the first and third levels. The second level molding and fan out structure includes back-to-back facing die, with a front surface of each die bonded to a redistribution layer.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/804,261, filed Jul. 20, 2015, which claims the benefit ofpriority from U.S. Provisional Patent Application Ser. No. 62/151,843filed on Apr. 23, 2015, the full disclosures of which are incorporatedherein by reference.

BACKGROUND

Field

Embodiments described herein relate to semiconductor packaging. Moreparticularly, embodiments relate to vertically stacked system in package(SiP) structures and methods of fabrication.

Background Information

The current market demand for portable and mobile electronic devicessuch as mobile phones, personal digital assistants (PDAs), digitalcameras, portable players, gaming, and other mobile devices requires theintegration of more performance and features into increasingly smallerspaces. As a result, various multiple-die packaging solutions such assystem in package (SiP) and package on package (PoP) have become morepopular to meet the demand for higher die/component density devices.

There are many different possibilities for arranging multiple die in anSiP. For example, vertical integration of die in SiP structures hasevolved into 2.5D solutions and 3D solutions. In 2.5D solutions themultiple die may be flip chip bonded on an interposer that includesthrough vias as well as fan out wiring. In 3D solutions multiple die maybe stacked on top of one another on an SiP substrate, and connected withoff-chip wire bonds or solder bumps.

In one implementation, a memory die or package (e.g., dynamicrandom-access memory (DRAM)) is stacked on top of a logic die or package(e.g., application-specific integrated circuit (ASIC)) or system on chip(SoC). As the market for portable and mobile electronic devices advanceslarger memory capability is required of the memory die or package.

SUMMARY

In an embodiment, a vertical stack SiP includes a first level dieencapsulated in a first level molding compound, a first redistributionlayer (RDL) on the encapsulated first level die, a second level diestack including a pair of back-to-back stacked die on the first RDL andencapsulated in a second level molding compound, a second RDL on theencapsulated second level die stack, a third level die on the second RDLand encapsulated in a third level molding compound, where the thirdlevel die is back facing toward the second RDL, and a third RDL on theencapsulated third level die.

In accordance with embodiments, the particular orientations of the dieare achieved within the SiP, which may be the result of particularpackaging methods. In an embodiment, the third RDL is directly on aconductive bump, such as a stud bump, of the third level die. In anembodiment, the third RDL is directly on a contact pad of the thirdlevel die. The third level die may be attached to the second RDL with adie attach film. The first level die may be front facing toward thefirst RDL, with the first RDL directly on a conductive bump of the firstlevel die. In accordance with embodiments, the pair of back-to-backstacked die may include a first-second level die bonded to the firstRDL, and a second-second level die, with the second RDL on thesecond-second level die. For example, the first-second level die may bebonded to the first RDL with solder, and the second RDL may be directlyon a conductive bump (e.g. stud bump) of the second-second level die.

The package levels may additionally include conductive pillars. Forexample, a plurality of second level conductive pillars can extend fromthe first RDL to the second RDL, and be encapsulated with the secondlevel molding compound. Similarly, a plurality of third level conductivepillars may extend from the second RDL to the third RDL, and beencapsulated with the third level molding compound. In an embodiment, aplurality of conductive bumps is formed on an opposite side of the thirdRDL from the third level die. In an embodiment, a plurality of firstlevel conductive pillars extend through the first level moldingcompound, and a second package is located on the first level moldingcompound and is electrically connected with and/or mechanicallysupported by the plurality of first level conductive pillars.

In an embodiment, a vertical stack SiP includes a first level volatilememory die encapsulated in a first level molding compound, a first RDLon the encapsulated first level volatile memory die, a second levelnon-volatile memory die stack including a pair of back-to-back stackednon-volatile memory die on the first RDL and encapsulated in a secondlevel molding compound, a second RDL on the encapsulated second levelnon-volatile memory die stack, a third level active die on the secondRDL and encapsulated in a third level molding compound, a third RDL onthe encapsulated third level active die. The vertical stack SiP mayinclude a plurality of the first level volatile memory die encapsulatedin the first level molding compound, with the first RDL on the pluralityof encapsulated first level volatile memory die. In an embodiment, thefirst level volatile memory die is a DRAM die, the back-to-back stackednon-volatile memory die are NAND die, and the third level active die isan SoC die.

In an embodiment, a method of forming a vertical stack SiP includesencapsulating a first level die on a carrier substrate with a firstlevel molding compound, forming a first RDL on the first level moldingcompound, encapsulating a second level die stack on the first RDL with asecond level molding compound, forming a second RDL on the second levelmolding compound, encapsulating a third level die on the second RDL witha third level molding compound, and forming a third RDL on the thirdlevel molding compound. For example, the first RDL may be formeddirectly on the first level die. In a first-second level die is bondedto the first RDL, and a second-second level die is attached to thefirst-second level die with a die attach film.

The fabrication methods may additionally include the integration ofconductive pillars. In an embodiment, a plurality of second levelconductive pillars are encapsulated with the second level moldingcompound, and the second RDL is formed directly on the second-secondlevel die in the second level die stack and the plurality of secondlevel conductive pillars. In an embodiment, a plurality of third levelconductive pillars are encapsulated with the third level moldingcompound, and the third RDL is formed directly on the third level dieand the plurality of third level conductive pillars.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view illustration of a plurality of diemounted on a carrier substrate in accordance with an embodiment.

FIG. 2 is a cross-sectional side view illustration of a plurality of ideencapsulated in a first level molding compound in accordance with anembodiment.

FIG. 3 is a cross-sectional side view illustration of a first RDL formedon a first level molding compound in accordance with an embodiment.

FIG. 4 is a cross-sectional side view illustration of conductive pillarsformed on a first RDL in accordance with an embodiment.

FIG. 5A is a cross-sectional side view illustration of a die mounted ona first RDL in accordance with an embodiment.

FIG. 5B is a close up cross-sectional side view illustration of a diebonded to a first RDL with polymer defined landing pads in accordancewith an embodiment.

FIG. 5C is a close up cross-sectional side view illustration of a diebonded to a first RDL with UBM defined landing pads in accordance withan embodiment.

FIG. 6 is a cross-sectional side view illustration of a die stackmounted on a first RDL in accordance with an embodiment.

FIG. 7 is a cross-sectional side view illustration of a second levelmolding and fan out structure on a first level molding and fan outstructure.

FIG. 8 is a cross-sectional side view illustration of a die mounted on asecond RDL and conductive pillars formed on the second RDL in accordancewith an embodiment.

FIG. 9A is a cross-sectional side view illustration of a third levelmolding and fan out structure on a second level molding and fan outstructure.

FIG. 9B is a cross-sectional side view illustration of a three layerstack structure prior to singulation of individual packages inaccordance with an embodiment.

FIG. 10 is a cross-sectional side view illustration of a verticallystacked SiP structure in accordance with an embodiment.

FIG. 11 is a cross-sectional side view illustration of a PoP structurein accordance with an embodiment.

FIG. 12 is a process flow illustrating a method of forming a verticallystacked SiP structure in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments describe vertically stacked SiP structures. In variousembodiments, description is made with reference to figures. However,certain embodiments may be practiced without one or more of thesespecific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of theembodiments. In other instances, well-known semiconductor processes andmanufacturing techniques have not been described in particular detail inorder to not unnecessarily obscure the embodiments. Reference throughoutthis specification to “one embodiment” means that a particular feature,structure, configuration, or characteristic described in connection withthe embodiment is included in at least one embodiment. Thus, theappearances of the phrase “in one embodiment” in various placesthroughout this specification are not necessarily referring to the sameembodiment. Furthermore, the particular features, structures,configurations, or characteristics may be combined in any suitablemanner in one or more embodiments.

The terms “front”, “back”, “to”, “between”, and “on” as used herein mayrefer to a relative position of one layer with respect to other layers.One layer “on” another layer or bonded “to” or in “contact” with anotherlayer may be directly in contact with the other layer or may have one ormore intervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

In one aspect, embodiments describe a vertical stack SiP. In anembodiment, a vertical stack SiP includes a first level die encapsulatedin a first level molding compound, a first redistribution layer (RDL) onthe encapsulated first level die, a second level die stack including apair of back-to-back stacked die on the first RDL and encapsulated in asecond level molding compound, a second RDL on the encapsulated secondlevel die stack, a third level die on the second RDL and encapsulated ina third level molding compound, and a third RDL on the encapsulatedthird level die. A plurality of second level conductive pillars mayelectrically connect the first RDL to the second RDL, and a plurality ofthird level conductive pillars may electrically connect the second RDLand the third RDL. In accordance with embodiments, conductive pillars(e.g. any of the first level, second level, third level, etc.) mayprovide mechanical support. For example, the mechanical support may beprovided in addition to electrical connection between components, orwithout providing electrical connection. In some embodiments, a portionof the conductive pillars within a package level are to provideelectrical connection and mechanical support, while another portion ofthe conductive pillars within the package level are to providemechanical support without electrical connection.

In one aspect, embodiments describe a vertical stack SiP that integratesmultiple types of memory die with a logic die (e.g. ASIC or SoC). In anembodiment, a vertical stack SiP includes separate molding levels for avolatile memory (e.g. DRAM, SRAM, pseudo SRAM, floating body, etc.),non-volatile memory (e.g. NAND, NOR, EPROM, EEPROM, MRAM, FRAM, PCM,etc.), and logic die. In an embodiment, a vertical stack SiP includes afirst level molding including one or more volatile memory die (e.g.DRAM), a second level molding including back-to-back stackednon-volatile memory die (e.g. NAND), and a third level molding includinga logic die (e.g ASIC or SoC).

In one aspect, embodiments described a vertical stack SiP that mayreduce the amount of real estate (e.g. x-y dimensions) on a circuitboard. It has been observed that certain non-volatile memory die (e.g.NAND) may have a larger x-y dimension footprint than certain volatilememory die (e.g. DRAM). For example, this may be attributed to anincreased memory capacity in mobile devices. In accordance withembodiments, non-volatile memory die for memory may have larger x-ydimensions than volatile memory die (e.g. used for cache). In accordancewith embodiments, a vertical stack SiP structure may include multiplefirst level die arranged side-by-side. In accordance with embodiments, avertical stack SiP structure may include multiple second level die witha large x-y dimension (relative to the other die in the SiP) stackedback-to-back within the vertical stack SiP. Additionally, fan out of theback-to-back stacked die can be accomplished with the use ofredistribution layers (RDLs) on opposite sides of the back-to-backstacked die. In this manner, the effect on total package height(z-height) can be mitigated with fan out using RDL, which can befabricated with substantially less thickness than for traditionalinterposers and wire bonding.

Referring now to FIG. 1 a cross-sectional side view illustration isprovided of a plurality of first level die 110 mounted on a carriersubstrate 102, such as a glass panel, silicon wafer, metal panel, etc.The carrier substrate 102 may include an adhesive (e.g. polymer) or tapelayer 104 for mounting the plurality of first level die 110. In anembodiment the first level die 110 are mounted onto the carriersubstrate with a film 112 such as a die attach film or epoxy bondingmaterial. In an embodiment, first level die 110 are memory die. In anembodiment, first level die 110 are volatile memory die such as DRAM,SRAM, pseudo SRAM, floating body, etc. In a specific embodiment, firstlevel die 110 are DRAM die.

In the embodiment illustrated in FIG. 1, the first level die 110 aremounted onto the carrier substrate 102 face up, such that the activeside including bumps 114 (e.g. stud bumps) is facing up. For example,stud bumps 114 may be copper stud bumps. Bumps 114 may be optional, andinstead may be exposed contact pads for the first level die 110. Inaccordance with embodiments, first level conductive pillars 120 mayoptionally be formed on the carrier substrate 102. The material ofoptional first level conductive pillars 120 can include, but is notlimited to, a metallic material such as copper, titanium, nickel gold,and combinations or alloys thereof. First level conductive pillars 120may be formed using a suitable processing technique, and may be formedof a variety of suitable materials (e.g. copper) and layers. In anembodiment, first level conductive pillars 120 are formed by a platingtechnique, such as electroplating using a patterned photoresist todefine the pillar structure dimensions, followed by removal of thepatterned photoresist layer. In an embodiment, the optional first levelconductive pillars 120 are formed prior to mounting of the first leveldie 110.

Referring now to FIG. 2, the plurality of first level die 110 andoptional first level conductive pillars 120 are then encapsulated in afirst level molding compound 122 on the carrier substrate 102. Forexample, the first level molding compound 122 may include athermosetting cross-linked resin (e.g. epoxy), though other materialsmay be used as known in electronic packaging. Encapsulation may beaccomplished using a suitable technique such as, but not limited to,transfer molding, compression molding, and lamination. Followingencapsulation with the first level molding compound 122, the structuremay optionally be additionally processed with a grinding (e.g. chemicalmechanical polishing) operation, etching operation, or patterned andetched to expose first level die 110 bumps 114, and optionally firstlevel conductive pillars 120. In an embodiment, top surfaces 115, 123 ofthe bumps 114 and first level molding compound 122 (and optionally topsurfaces 121 of first level conductive pillars 120) are coplanar after agrinding or etching operation. In an embodiment, bumps 114 may bereplaced with contact pads of the first level die 110, which may beexposed, for example, by etching or laser drilling the first levelmolding compound 122.

Referring now to FIG. 3 a first redistribution layer (RDL) 130 is formedon the first level molding compound 122 and the exposed surfaces 115 ofbumps 114 (or contact pads), and optionally exposed surfaces 121 of thefirst level conductive pillars, when present. The first RDL 130 mayinclude a single redistribution line 132 or multiple redistributionlines 132 and dielectric layers 134. The first RDL 130 may be formed bya layer-by-layer process, and may be formed using thin film technology.In an embodiment, the first RDL 130 has a total thickness of less than50 μm, or more specifically less than 30 μm, such as approximately 20μm. In an embodiment, first RDL 130 includes embedded redistributionlines 132 (embedded traces). For example, the redistribution lines 132may be created by first forming a seed layer, followed by forming ametal (e.g. copper) pattern. Alternatively, redistribution lines 132 maybe formed by deposition (e.g. sputtering) and etching. The material ofredistribution lines 132 can include, but is not limited to, a metallicmaterial such as copper, titanium, nickel, gold, and combinations oralloys thereof. The metal pattern of the redistribution lines 132 isthen embedded in a dielectric layer 134, which is optionally patterned.The dielectric layer(s) 134 may be any suitable material such as anoxide, or polymer (e.g. polyimide).

In the embodiment illustrated, redistribution lines 132 are formeddirectly on the top surfaces 115 of bumps 114 (or contact pads). Morespecifically, contact pads 135 of the redistribution lines 132 of thefirst RDL 130 are formed directly on the bumps 114 of first level die110. Together, the first RDL 130, and molded first level die 110 mayform a first level molding and fan out 125.

Following the formation of the first RDL 130 a plurality of second levelconductive pillars 140 may be formed on the first RDL 130 as illustratedin FIG. 4. Second level conductive pillars 140 may be formed similarly,and of the same materials as described above with regard to the optionalfirst level conductive pillars 120.

Referring now to FIG. 5A one or more second level die 142 are mounted onthe first RDL 130. In an embodiment, the second level die 142 is anon-volatile memory die, such as (e.g. NAND, NOR, EPROM, EEPROM, MRAM,FRAM, PCM, etc.). In a specific embodiment, second level die 142 is aNAND die. In an embodiment, second level die 142 is wider, with largerx-y area, than either of the first level die 110. In the embodimentillustrated in FIG. 5A, second level die 142 is front facing toward thefirst RDL 130 and is attached to landing pads or underbump metallurgy(UBM) pads of the first RDL 130 with conductive bumps, such as studbumps, solder bumps, or stud bumps with solder tips. In an embodiment,the back side of the second level die 142 does not include anyconductive contacts (e.g. stud bumps, solder bumps, etc.).

The landing pads or UBM pads can be formed in the first RDL 130 in avariety of ways. FIG. 5B is a close up illustration of a second leveldie 142 bonded to a first RDL in which landing pad openings have beendefined by openings in a dielectric layer 134. In the particularembodiment illustrated, the second level die 142 bumps include studbumps 144 with solder tips 146. FIG. 5C is a close up illustration of asecond level die 142 including stud bumps 144 with solder tips 146bonded to a first RDL in which the landing pads are defined by UBM pads136. Referring now back to FIG. 5A, following mounting of the secondlevel die 142 to the first RDL 130, an underfill material 150 mayoptionally be applied to between the second level die 142 and first RDL130.

Referring now to FIG. 6, a second-second level die 142 is attached tothe first-second level die 142. In the particular embodimentillustrated, a back side of the second-second level die 142 is attachedto a back side of the first-second level die 142 in a back-to-backarrangement. The second level die 142 may be attached to each otherusing a die attach film (DAF) 148, for example. DAF 148 may be anadhesive material, and may optionally be thermally conductive. DAF mayoptionally be cured after die attachment through chemical, thermal orultraviolet light, for example.

In an embodiment, the first (e.g. top in FIG. 6) and second (e.g. bottomin FIG. 6) second level die 142 are identical. For example, each secondlevel die 142 may be the same NAND die. In an embodiment, the stackedsecond level die 142 are the same, with one exception being amodification to the stud bumps. For example, the top second level die142 (as shown in FIG. 6) may include stud bumps 144 without solder tips(or alternatively contact pads where stud bumps are not present), whilethe bottom second level die 142 (as shown in FIG. 6) includes stud bumps144 with solder tips 146, as illustrated in FIGS. 5B-5C.

Referring now to FIG. 7, second level die 142 stack and second levelconductive pillars 140 are encapsulated in a second level moldingcompound 152 on the carrier substrate 102. Referring briefly to FIG. 9B,the second level molding compound 152 may optionally surround the firstlevel molding compound 122, though this is not required. The secondlevel molding compound 152 may be formed similarly as, and from the samematerial as the first level molding compound 122. Followingencapsulation with the second level molding compound, the structure mayoptionally be processed with a grinding operation, etching operation, orpatterned and etched to expose the top second level die 142 bumps 144(or contact pads if bumps are not present), and second level conductivepillars 140. In an embodiment, the top surfaces 145 of the bumps 144,the top surface 153 of the second level molding compound 152, and thetop surfaces 141 of the second level conductive pillars 140 are coplanarafter a grinding or etching operation.

A second redistribution layer (RDL) 160 is then formed on the secondlevel molding compound 152, the exposed surfaces 145 of bumps 144 (orcontact pads), and the exposed surfaces 141 of the second levelconductive pillars 140. The second RDL 160 may include a singleredistribution line 162 or multiple redistribution lines 162 anddielectric layers 164. The second RDL 160 may be formed by alayer-by-layer process, and may be formed using thin film technology. Inan embodiment, the second RDL 160 has a total thickness of less than 50μm, or more specifically less than 30 μm, such as approximately 20 μm.In an embodiment, second RDL 160 includes embedded redistribution lines162 (embedded traces). For example, the redistribution lines 162 may becreated by first forming a seed layer, followed by forming a metal (e.g.copper) pattern. Alternatively, redistribution lines 162 may be formedby deposition (e.g. sputtering) and etching. The material ofredistribution lines 162 can include, but is not limited to, a metallicmaterial such as copper, titanium, nickel, gold, and combinations oralloys thereof. The metal pattern of the redistribution lines 162 isthen embedded in a dielectric layer 164, which is optionally patterned.The dielectric layer(s) 164 may be any suitable material such as anoxide, or polymer (e.g. polyimide).

In the embodiment illustrated, redistribution lines 162 are formeddirectly on the top surfaces 145 of bumps 144 (or contact pads wherebumps are not present). More specifically, contact pads 165 of theredistribution lines 162 of the second RDL 160 are formed directly onthe bumps 144 of the top second level die 142. Together, the second RDL160, and molded second level stacked die 142 may form a second levelmolding and fan out 155. Redistribution lines 162 may also be formeddirectly on the surfaces 141 of the plurality of second level conductivepillars 140.

Following the formation of the second RDL 160 a plurality of third levelconductive pillars 170 may be formed on the second RDL 160 asillustrated in FIG. 8. Third level conductive pillars may be formedsimilarly, and of the same materials as described above with regard tothe optional first level conductive pillars 120.

Still referring to FIG. 8, one or more third level die 172 are mountedon the second RDL 160. For example, the one or more third level die 172may be mounted after the formation of third level conductive pillars170. In an embodiment, the third level die 172 is a logic die, such asan ASIC or SoC. In an specific embodiment, third level die 172 is an SoCdie. As shown in FIG. 8, third level die 172 may be back facing to thesecond RDL 160. In such an arrangement, the third level die 172 may beattached to the second RDL 160 with a DAF 178, similar to DAF 148described above. Third level die 172 may include bumps 174, such as studbumps (e.g. copper stud bumps). Alternatively, third level die 172 mayinclude exposed contact pads in place of bumps 174.

Referring now to FIG. 9A, third level die 172 and third level conductivepillars 170 are encapsulated in a third level molding compound 182 onthe carrier substrate 102. Referring briefly to FIG. 9B, the third levelmolding compound 182 may optionally surround the first and second levelmolding compounds 122, 152, though this is not required. The third levelmolding compound 182 may be formed similarly as, and from the samematerial as the first and second level molding compounds 122, 152.Following encapsulation with the third level molding compound, thestructure may optionally be processed with a grinding operation, etchingoperation, or patterned and etched to expose the third level die 172bumps 174, (or contact pads) and third level conductive pillars 170. Inan embodiment, the top surfaces 175 of the bumps 174, the top surface183 of the third level molding compound 182, and the top surfaces 171 ofthe third level conductive pillars 170 are coplanar after a grinding oretching operation.

A third redistribution layer (RDL) 190 is then formed on the third levelmolding compound 182, the exposed surfaces 175 of bumps 174 (or contactpads), and the exposed surfaces 171 of the third level conductivepillars 170. The third RDL 190 may include a single redistribution line192 or multiple redistribution lines 192 and dielectric layers 194. Thethird RDL 190 may be formed by a layer-by-layer process, and may beformed using thin film technology. In an embodiment, the third RDL 190has a total thickness of less than 50 μm, or more specifically less than30 μm, such as approximately 20 μm. In an embodiment, third RDL 190includes embedded redistribution lines 192 (embedded traces). Forexample, the redistribution lines 192 may be created by first forming aseed layer, followed by forming a metal (e.g. copper) pattern.Alternatively, redistribution lines 192 may be formed by deposition(e.g. sputtering) and etching. The material of redistribution lines 192can include, but is not limited to, a metallic material such as copper,titanium, nickel, gold, and combinations or alloys thereof. The metalpattern of the redistribution lines 192 is then embedded in a dielectriclayer 194, which is optionally patterned. The dielectric layer(s) 194may be any suitable material such as an oxide, or polymer (e.g.polyimide).

In the embodiment illustrated, redistribution lines 192 are formeddirectly on the top surfaces 175 of bumps 174. More specifically,contact pads 195 of the redistribution lines 192 of the third RDL 190are formed directly on the bumps 174 (or contact pads) of die 172.Together, the third RDL 190, and molded third level die 172 may form athird level molding and fan out 185. Following the formation of thethird RDL 190 a plurality of conductive bumps 198 (e.g. solder bumps, orstud bumps) may be formed on the third RDL 190.

Referring now to FIG. 9B, a cross-sectional side view illustration isprovided of the three layer (or three level) stack structure inaccordance with an embodiment prior to singulation of individualpackages, in which the dotted lines illustrate singulation lines ofindividual packages. In an embodiment, edges of the molding compounds122, 152 may be notched to accommodate a molding cavity for use duringencapsulation. The notched area may sequentially be trimmed duringsingulation. The particular embodiment illustrated in FIG. 9B isexemplary, and a variety of molding configurations are possible. Thesame or different molding cavities may be used for the different moldinglevels. Additionally, the molding cavities can have the same ordifferent depths (height), and area. In an embodiment, same moldingcavity can be used for all molding levels.

FIG. 10 is a cross-sectional side view illustration of a verticallystacked SiP structure after removal of the carrier substrate and packagesingulation. In an embodiment, a vertically stacked SiP includes a firstlevel die 110 encapsulated in a first level molding compound 122, afirst redistribution layer (RDL) 130 on the encapsulated first level die110, a second level die stack including a pair of back-to-back stackeddie 142 on the first RDL 130 and encapsulated in a second level moldingcompound 152, a second RDL 160 on the encapsulated second level diestack, a third level die 172 on the second RDL 160 and encapsulated in athird level molding compound 182, and a third RDL 190 on theencapsulated third level die 172. A plurality of second level conductivepillars 140 may electrically connect the first RDL 130 to the second RDL160, and a plurality of third level conductive pillars 170 mayelectrically connect the second RDL 160 and the third RDL 190. As shown,the third level die 172 is back facing toward the second RDL 160 (e.g.there are no conductive contacts on the back side of the third level die172 facing the second RDL 160). In such a configuration, there is nodirect electrical connection between the third level die 172 and thesecond RDL 160. For example, the third level die 172 may be attached tothe second RDL with a die attach film 178. The third RDL 190 may bedirectly on a conductive bump 174 (e.g. stud bump) of the third leveldie 172. In an embodiment, an electrical path between the third leveldie 172 and the second RDL 160 runs through the third RDL 190 and thirdlevel conductive pillars 170 to the second RDL 160.

As shown, the first level die 110 is front facing toward the first RDL.The first RDL 130 may be directly on a conductive bump 114 (e.g. studbump) of the first level die 110. There may be a plurality ofside-by-side first level die 110. This may reduce total z-height of thepackage as opposed to vertically stacking the first level die 110. In anembodiment, the one or more first level die 110 are DRAM die.

The pair of back-to-back stacked die 142 may include a first-secondlevel die 142 bonded to the first RDL 130, and a second-second level die142, where the second RDL 160 is on the second-second level die 142. Asshown, the first-second level die 142 may be bonded to the first RDL 130with solder. The second RDL 160 may be directly on a conductive bump 144(e.g. stud bump) of the second-second level die 142. The second-secondlevel die 142 may be attached to the first-second level die 142 with adie attach film 148. In an embodiment, the pair of back-to-back stackeddie 142 are non-volatile memory die, such as NAND die. In accordancewith embodiments, the NAND die are stacked back-to-back, as opposed toside-by-side due to their comparatively large size. Thus, total packagesize, both x-y and z-height may be reduced using the back-to-backstacking configuration within the middle of the package.

A plurality of second level conductive pillars 140 may extend from thefirst RDL 130 to the second RDL 160, and be encapsulated within thesecond level molding compound 152. A plurality of third level conductivepillars 170 may extend from the second RDL 160 to the third RDL 190, andbe encapsulated within the third level molding compound 182. A pluralityof conductive bumps 198 may be formed on an opposite side of the thirdRDL 190 from the third level die 172. In an embodiment, the third leveldie 172 is attached to the second RDL 160 with a die attach film 178. Inan embodiment, the one or more first level die 110 is a volatile memorydie (e.g. DRAM), the pair of back-to-back stacked die 142 arenon-volatile memory die (e.g. NAND), and the third level die is a logicdie (e.g. SoC).

Still referring to FIG. 10, in an embodiment a passivation layer 200 isoptionally formed over the first level molding compound 122 and firstlevel die 110. For example, passivation layer 200 may be formed bylamination. In one embodiment the passivation layer 200 is formed afterremoval of the carrier substrate 102 and prior to singulation of the SiPstructures. In another embodiment, the passivation layer 200 can beformed on the carrier substrate 102 illustrated in FIG. 1 prior toformation of the optional first level conductive pillars 120 and/orattachment of the first level die 110. For example, passivation layer200 can be formed over the adhesive (e.g. polymer) or tape layer 104.

FIG. 11 is a cross-sectional side view illustration of a PoP structurein accordance with an embodiment. As described with regard to FIGS. 1-2first level conductive pillars 120 are optionally formed on the carriersubstrate 102, and encapsulated with a first level molding compound 122.Upon removal of the carrier substrate 102, the first level conductivepillars 120 may be exposed. Additional processing such as grinding oretching may also be performed to expose the first level conductivepillars. As shown in FIG. 11, in an embodiment a second package 210 maybe in electrical connection (e.g. bonded to with conductive bumps 198)the first level conductive pillars 120 extending through the first levelmolding compound 122 of the vertically stacked SiP structure to form aPoP structure.

FIG. 12 is a process flow illustrating a method of forming a verticallystacked system in package in accordance with an embodiment. At block1210 a first level die is encapsulated on a carrier substrate with afirst level molding compound, for example, similarly as described withregard to FIG. 2. At block 1220 a first RDL is formed on the first levelmolding compound, for example, similarly as described with regard toFIG. 3. In an embodiment, the first RDL is formed directly on the firstlevel die. At block 1230 a second level die stack is encapsulated on thefirst RDL with a second level molding compound, for example, similarlyas described with regard to FIGS. 4-6. In an embodiment, the secondlevel die stack is formed by bonding a first-second level die to thefirst RDL, and attaching a second-second level die to the first-secondlevel die with a die attach film. In an embodiment, a plurality ofsecond level conductive pillars formed on the first RDL are encapsulatedwith the second level molding compound. At block 1240 a second RDL isformed on the second level molding compound, for example, similarly asdescribed with regard to FIG. 6. In an embodiment, the second RDL isformed directly on the second-second level die in the second level diestack and the plurality of second level conductive pillars. At block1250 a third level die is encapsulated on the second RDL with a thirdlevel molding compound, for example, similarly as described with regardto FIG. 9A. In an embodiment, a plurality of third level conductivepillars formed on the second RDL are encapsulated with the third levelmolding compound. At block 1260 a third RDL is formed on the third levelmolding compound. The third RDL may be formed directly on the thirdlevel die and the plurality of third level conductive pillars. Aplurality of conductive bumps (e.g. solder balls) may be formed (e.g.dropped) on the third RDL, and the carrier substrate may then bereleased. For example, this may result in a vertically stacked SiPstructure similar to that described with regard to FIG. 10. Where firstlevel conductive pillars are present, a second package can be stacked onthe vertically stacked SiP structure to form a PoP structure, similar tothat described with regard to FIG. 11.

In utilizing the various aspects of the embodiments, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a stacked system inpackage structure. Although the embodiments have been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the appended claims are not necessarily limitedto the specific features or acts described. The specific features andacts disclosed are instead to be understood as embodiments of the claimsuseful for illustration.

What is claimed is:
 1. A vertical stack system in package (SiP)comprising: a pair of first level dies encapsulated in a first levelmolding compound; a first redistribution layer (RDL) on the encapsulatedpair of first level dies; a second level die stack including a pair ofback-to-back stacked dies on the first RDL and encapsulated in a secondlevel molding compound; a second RDL on the encapsulated second leveldie stack; a third level die on the second RDL and encapsulated in athird level molding compound, wherein the third level die is back facingtoward the second RDL, wherein the third level die is attached directlyto the second RDL with a die attach film that is in direct contact withthe third level die and the second RDL; and a third RDL on theencapsulated third level die; wherein each of the first level dies is afirst type of die and each of the back-to-back stacked dies is a secondtype of die that is different than the first type of die, and each ofthe back-to-back stacked dies have larger x-y dimensions than each ofthe first level dies.
 2. The vertical stack SiP of claim 1, wherein thethird RDL is directly on a stud bump of the third level die.
 3. Thevertical stack SiP of claim 1, wherein the third RDL is directly on acontact pad of the third level die.
 4. The vertical stack SiP of claim1, wherein each of the first level dies is front facing toward the firstRDL and the first RDL is directly on a conductive bump for each of thefirst level dies.
 5. The vertical stack SiP of claim 1, wherein the pairof back-to-back stacked dies includes a first-second level die bonded tothe first RDL, and a second-second level die, wherein the second RDL ison the second-second level die.
 6. The vertical stack SiP of claim 5,wherein the first-second level die is bonded to the first RDL withsolder.
 7. The vertical stack SiP of claim 6, wherein the second RDL isdirectly on a stud bump of the second-second level die.
 8. The verticalstack SiP of claim 5, further comprising a plurality of second levelconductive pillars extending from the first RDL to the second RDL,wherein the plurality of second level conductive pillars areencapsulated with the second level molding compound.
 9. The verticalstack SiP of claim 8, further comprising a plurality of third levelconductive pillars extending from the second RDL to the third RDL,wherein the plurality of third level conductive pillars are encapsulatedwith the third level molding compound.
 10. The vertical stack SiP ofclaim 9, further comprising a plurality of conductive bumps on anopposite side of the third RDL from the third level die.
 11. Thevertical stack SiP of claim 9, further comprising: a plurality of firstlevel conductive pillars extending through the first level moldingcompound; and a second package on the first level molding compound, andelectrically connected with the plurality of first level conductivepillars.
 12. The vertical stack SiP of claim 1, wherein the pair ofback-to-back stacked dies includes a first-second level die bonded tothe first RDL, and a second-second level die, wherein the first-secondlevel die is bonded to the first RDL with solder and the second RDL isdirectly on a stud bump of the second-second level die.
 13. The verticalstack SiP of claim 1, wherein the third level die is a third type of diethat is different than both the first type of die and the second type ofdie.
 14. The vertical stack SiP of claim 1 wherein: the first type ofdie is selected from the group consisting of DRAM, SRAM, pseudo SRAM,and floating body; the second type of die is selected from the groupconsisting of NAND, NOR, EPROM, EEPROM, MRAM, FRAM, and PCM; and thethird type of die is selected from the group consisting of ASIC and SoC.